Merge pull request #3 from gully/master

I think the standard deviation and variance were switched here, double c...
Fig 1.13 errata
Merge pull request #4 from gully/master

add erratum for chapter 4
added error on p. 427
added counting up to 8 error on p. 47/50
Add appendix errata
Add SDSS errata
Add SDSS errata
Add SDSS errata
change wording
verified the incorrect URL
fix typo
Merge pull request #8 from weaverba137/master

Errata for Chapter 1 & Appendix
small edits of request #8 from weaverba137
implemented #9
implemented #9
implemented #11
implemented #10
Add note about chapter 5 equation.
Merge pull request #12 from weaverba137/master

Add note about chapter 5 equation.